The invention relates to extending the inherent signal-to-noise (S/N) advantage of high field MR to human imaging and spectroscopy, doing so by using a distributed circuit approach to designing RF volume coils, which approach wavelength dimensions. Performance of conventional lumped element designs succumbs to: 1) non uniform current distributions resulting in decreased homogeneity, decreased fill factor, and increased electric field losses, 2) decreased conductor skin depths resulting in decreased Q and increased ohmic losses, 3) self resonance below or too close to the desired frequency of operation and 4) increased electromagnetic radiative losses.
At higher frequencies the phase change due to finite propagation velocity of transmit and receive signals on coil conductors is no longer negligible. To preserve coil performance above 100 MHz, coil circuits must be distributed, and the distributed nature of the patient loaded coil must be considered.
More specifically in the present invention, lumped elements are replaced by transmission line and cavity elements, in the design of which lumped element circuit theory has been replaced by transmission line, i.e., transverse electromagnetic (TEM) theory. DC circuit and field analysis is replaced with fully time-dependent AC analysis for the coil and the human load. High frequency coils designed by the high frequency methods presented herein, have optimum desired B.sub.1 field characteristics minimized coil losses and maximized self resonance. The inherent SIN advantage of high field MR is thereby realized in clinical images and spectra.
Lumped transmission line elements have been proposed heretofore for use in high frequency MR coil circuits. Thus, in 1976 Schneider and Dullenkopf proposed their slotted tube resonator probe head (1). More recently, Barfuss et al., used capacitively shortened, half wave slotted tube lines for 170 MHz (4 T) human body coils (2). Roschmann minimized the electric field loss problems of these resonator designs by replacing the unshielded, lumped element, slotted tube with a "tube" of half wave coaxial transmission line elements (3). The present invention provides further improvements of this transmission line (TEM) resonator in the form of variable tuning, double tuning, matching, and quadrature phasing (4, 5, 6).
In the previously known NMR coils, an RF shield was provided around the coil to stop or shield all signals at the radio frequency at which the RF coil resonates and to also pass unimpeded the switched DC gradient fields used to magnetically localize an image slice or volume.
In these previously known shields for NMR coils, such coils used a thick copper or conductive layer that retarded the switch gradient field induced eddy current by cutting or etching breaks or gaps in the conductive layer to interrupt or break up any eddy current flux path. This approach also interrupted the RF flux path making the shield inefficient or "leaky". Additionally, the gaps in these prior art shields allowed some RF to pass through the shield unshielded.